1. Field of the Invention
The present invention relates to a device for determining at least one characteristic point of a joint which joins together two limbs and is to be measured orthopedically, the device comprising a first coupling element which may be attached to a first limb adjoining the joint, a second coupling element which may be moved relative to the first coupling element and may be attached to a second limb adjoining the joint, and a measuring device for determining the position of the first coupling element relative to the second coupling element.
2. Description of the Background of the Invention
In orthopedic technology, there are various applications in which a joint of human extremities is to be measured. It is particularly necessary in the field of joint orthotics or prosthetics to determine the dynamic behavior of joints of human extremities. Thus, for example, it is necessary to specifically adapt outhouses, which are an external stabilization aid for the mechanical and functional stabilization of joints, to the respective joint of the patient which is to be supported. A preferred application of outhouses are knee joint orthoses. These are used to stabilize the knee joint in the case of abnormal weakening or weakening as a result of an operation in the same way as they are used by competitive athletes to avoid strains. The knee joint in particular does not behave, however, like a simple hinge joint with a singular axis of rotation, but rather like a polycentric joint with a plurality of pivots, the pivot positions of which constantly change with respect to one another during a relative movement of thigh and lower leg. Although in the meantime there have been various attempts at using in orthotics and prosthetics polycentric joints which imitate more or less effectively the actual behavior of the respective human joint to be supported, monocentric joints basically continue to be used in orthoses and prostheses.
Monocentric and polycentric orthoses and prostheses have to be adapted individually to the respective patient. The result of mismatching an orthosis may be, for example, that said orthosis does not ensure the required mechanical and functional stabilization of the respective joint. Furthermore, a mismatch of an orthosis may even result in secondary damage to the joint, because this joint is additionally stressed by the resistance produced by the orthosis per se to stabilize the joint, against certain movements. To adapt the orthosis or prosthesis, therefore, a compromise pivotal point and/or a polar curve reflecting the path of instantaneous pivotal points at various bending angles of the joint have been determined hitherto. The orthosis or prosthesis is adjusted by an orthopaedic technician according to the determined compromise pivotal point or the polar curve. The Nietert method, with which a person skilled in the art is generally acquainted, is used in particular for determining the compromise pivotal point. This method produces a range within which the compromise pivotal point is to be expected. However, it has been found that a determination of this type is not sufficiently accurate, for example in reliably adapting an orthosis to every patient while avoiding mismatches.
Formerly, axes of rotation in joints were mainly measured in expensive clinical studies. In so doing, markers were preferably placed in bones of cadaver preparations and the movement thereof was traced photographically and radiologically. These measures lead to general findings in respect of the position of compromise pivotal point and polar curves on various joints, but do not assist individual adaptation of an orthosis or prosthesis to the respective patient.
Goniometers have long been used for specifically adapting orthoses to patients. A goniometer allows, for example, the extent of movement of a joint to be determined, although the use of a goniometer provides no information about the position of the axes of rotation. Reference is made to WO94/07108 concerning the construction and mode of operation of an orthopedically used goniometer. The prior art according to U.S. Pat. No. 4,306,571 also describes the possibility of using a multiple goniometer for obtaining data while stressing a knee joint.
Another approach for measuring patients is known from European patent application EP 1 454 584 A1 . In this case, a patient is positioned on a support plate mounted on measuring cells and is measured by laser beams. This prior art provides in particular the production of a dynamic effect line for a patient, based on the measurement result obtained via the support plate, and a prosthesis or orthosis may be adapted using this dynamic effect line.
DE 10014397 A1 describes a device for measuring angles of joints. The device consists of two rails connected to a goniometer by a preferably fixable pivot. Positioned on these rails are contact elements, of which at least one may be moved axially to the rail. To use the device, the limbs of the body which are to be measured and are connected by a common joint are brought into the position to be measured. The contact elements are then positioned on the body limbs at defined anatomical points such that the existing angle between the rails corresponds to the angle of the joint between the body limbs. Fixing the pivot of the device facilitates the removal of the device from the body and reading off the angle of the joint, without changing the angle between the rails.
Furthermore, U.S. Pat. No. 4,436,099 describes a similar device for measuring the relative angular displacement between two body limbs connected by a common joint. In this device, two rigid rods are connected to a load cell, one of the rods being rotatable by a joint attached to the cell and the other rod being attached to the cell. The rods are each attached to one of the limbs by holding means in which the rods are able to rotate and may be moved parallel to the longitudinal axis of the relevant limb. Located in the load cell is a circuit which produces an electrical signal which is an indication of the angular displacement between the rigid rods, i.e. also of the angular displacement between the limbs. The electrical signal is further conveyed via a cable to a counter and a display. The display is appropriately installed at a distance from the body to be measured, so that it is easily possible to read the measured angular displacement between the limbs.
The document U.S. Pat. No. 6,692,447 B1 describes a system for determining pivot centers for proximal and intermediary articulations. For that purpose a signal marker is fixed to a bone, in order to determine pivot points of such proximal and intermediary articulations.
The object of the present invention is to provide a device and a method of the type mentioned at the outset, using which it is possible, with simple handling, to determine with sufficient accuracy a characteristic point of a patient's joint to be measured orthopedically.
This object is achieved by a device of the type mentioned at the outset, in which the measuring device has at least one reference point which is rigidly connected to one of the first and second coupling elements, and also has a reference point detection device which is rigidly connected to the respective other coupling element of the first and second coupling elements, the reference point detection device detecting a plurality of positions of the reference point relative to the reference point detection device where there is a movement of the first and second limbs towards one another and determining from this the at least one characteristic point.
The device according to the invention may be attached directly to the limbs joined together by the joint to be measured, i.e. it may be held thereon or secured thereto. When the coupling elements of the device according to the invention have been attached to the limbs, the limbs may be moved relative to one another while bending the joint, the joint performing its natural movements. In this manner it is possible to detect the relative movement of the two limbs with respect to one another in a substantially unstressed position of the joint as well as when the joint is fully loaded using the device according to the invention. This is carried out in that the at least one reference point rigidly connected to one of the first and second coupling elements moves with the respectively associated coupling element corresponding to the movement of the joint. This movement is detected selectively by the reference point detection device at least at a plurality of points. The characteristic point, for example a compromise centre of rotation, may then be determined from these instantaneous positions of the at least one reference point. The findings thus obtained for the position of the characteristic point, in particular the compromise centre of rotation, may then be used by an orthopaedic surgeon or an orthopaedic technician to adapt an orthosis or a prosthesis. Thus, for example, to adapt a monocentric orthosis, the pivot axis thereof may be made to correspond to the compromise axis of rotation determined by the device according to the invention in order to utilize in the best possible manner the mechanical stabilization function of the orthosis for the joint to be stabilized. Furthermore, the device according to the invention may determine a characteristic point, for example a compromise centre of rotation, for a healthy joint to be measured and a joint prosthesis may be correspondingly adapted for an extremity to be provided with a prosthesis, according to the measurement data obtained.
The invention therefore provides the possibility of measuring a joint orthopedically with sufficient accuracy and with relatively simple technical means and with a low measuring complexity, it being possible to further use the obtained measurement data for the orthotic or prosthetic care of a patient.
A development of the invention provides that the reference point detection device detects a movement path of the respective reference point from the plurality of positions of the reference point. It should be noted that with an increase in number of detected positions of the reference point, the findings obtained via the joint to be measured orthopedically become more detailed and thus a more accurate determination of the characteristic point of this joint is possible. The detected movement path may be a two-dimensional curve (projected in a plane) or a three-dimensional curve.
In a preferred embodiment of the invention, a processing system is provided which is coupled or may be coupled to the reference point detection device for transmitting data. The processing system processes in a predetermined manner the data obtained from the reference point detection device. The processing system may be configured separately from the device which may be positioned on or attached to the joint, or may also be configured integrally therewith. A development of the invention preferably provides that the processing system determines the at least one characteristic point as at least one virtual pivotal point for the joint to be measured from the data received from the reference point detection device, in particular from the movement path of the respective reference point. The determined virtual pivotal point is often also referred to by experts as a compromise pivotal point, a compromise centre of rotation or a compromise axis of rotation.
Particularly for the adjustment of polycentric joints, detailed investigations are necessary which go beyond determining a singular characteristic point, for example of the comprise centre of rotation. In this connection, a development of the invention provides that the processing system determines a plurality of characteristic points, preferably as a polar curve, for the joint to be measured from the data received from the reference point detection device, in particular from the movement path of the respective reference point. The polar curve (also called centrode) reflects the path of the instantaneous pivotal points at various bending angles of the respective joint. A polycentric orthosis or prosthesis may then be adjusted, for example, according to the determined polar curve, so that the polar curve of the orthosis corresponds substantially to the polar curve of the joint to be provided with an orthosis or that the polar curve of the polycentric prosthesis for an extremity, to be provided with a prosthesis, of a patient is adapted accordingly to the polar curve of the healthy joint of the respective other extremity of the patient to avoid asymmetric strains on the patient.
As already described above, the findings, obtained via the device according to the invention, on the characteristic point of the joint to be measured orthopedically are used in a preferred application to adjust an orthosis for the joint. This is carried out in the easiest way in that the orthosis is directly adapted in the immediate vicinity of the joint. To this end it is necessary for the determined characteristic point, for example the virtual pivotal point (compromise pivotal point) to be made visible to the joint to be measured orthopedically. For this purpose, a development of the invention provides an optical output means by which the at least one determined characteristic point of the joint may be indicated optically on the joint to be measured. This may be carried out, for example, in that the output means comprises an angled arrangement of a plurality of light sources which are able to emit preferably directed light beams, it being possible to indicate the at least one determined virtual pivotal point by a point of intersection of the two both light beams. Alternatively, the determined characteristic point of the joint may also be indicated by one or more laser beams.
As an alternative to indicating the at least one determined virtual pivotal point by light beams crossing one another in a point of intersection, it is also possible for the output means to have an arrangement of a plurality of light sources which indicate contact points in particular for positioning a gauging arrangement, in particular a ruler arrangement, it being possible to indicate the at least one determined virtual pivotal point on the joint by suitable positioning of the ruler arrangement. Individual light points are thus able to indicate coordinates for the virtual pivotal point so that it is possible to indicate and mark the determined virtual pivotal point on the joint using the rulers by the contacting of one or more rulers by an orthopaedic technician or the like using the indicated coordinates. Alternatively, the orthopaedic technician can mark out by eye the virtual pivotal point on the joint without rulers using the indication points.
As an alternative to an optical indication means using light sources, a development of the invention provides an electromechanical output means which has an electromechanically displaceable marking, it being possible to indicate the at least one determined virtual pivotal point by suitably positioning the electromechanically displaceable marking on the joint. For example, the marking may be displaced electromechanically within a specific field via one or more linear adjusting elements and the marking can then remain at the at least one determined virtual pivotal point for marking by electromechanically activating the adjusting elements according to the determined characteristic point.
An advantageous development of the invention provides a marking device, in particular a pencil, using which it is possible to make a color marking on the joint at the at least one determined virtual pivotal point after a corresponding electromechanical displacement of the marking. The device according to the invention is thus able to automatically make a marking on the patient's joint which is to be provided with an orthosis or prosthesis, which marking may then be used later on for adapting an orthosis or prosthesis.
The foregoing has generally described how it is possible to detect the actual position of the reference point using the reference point detection device. This may be carried out, for example, in that the reference point detection device optically detects the position of the at least one reference point. As an alternative or additionally to this, it may be provided that the reference point detection device electronically detects the position of the at least one reference point. For example, the position of the reference point may be determined magnetically via Hall sensors. Alternatively, it is also possible to join together the two coupling elements via a multijoint arrangement, it being possible to determine the exact relative position of the first coupling element relative to the second coupling element via rotary or linear position transmitters. Linear or rotary position transmitters of this type may be, for example, corresponding linear or rotary potentiometers. Basically, it should be noted that every technical possibility of determining the relative position of the first coupling element relative to the second coupling element, in particular of determining the relative position of at least one reference point rigidly connected to a coupling element, relative to the other coupling element, lies within the scope of the invention, provided it that it affords adequate accuracy.
The measuring accuracy of the device according to the invention may be further increased in that at least two mutually spaced apart reference points, which are rigidly connected to one of the first and second coupling elements, are monitored. In this respect, the reference point detection device detects a plurality of positions of the reference points, preferably the movement paths of the at least two reference points relative to the reference point detection device, during a movement of the first and second limb with respect to one another.
The invention also relates to a method for determining at least one characteristic point of a joint which joins together two limbs and is to be measured orthopedically, using a device, in particular of the type described above, the method comprising the following steps:                positioning a first coupling element on a first limb adjoining the joint,        positioning a second coupling element, movable relative to the first coupling element, on a second limb adjoining the joint, and        determining the position of the first coupling element relative to the second coupling element using a measuring device.        
This method provides according to the invention that the measuring device has at least one reference point rigidly connected to one of the first and second coupling elements, and also has a reference point detection device rigidly connected to the respective other coupling element of the first and second coupling elements, the reference point detection device detecting a plurality of positions of the reference point relative to the reference point detection device during a movement of the first and second limb relative to one another and the at least one characteristic point being determined therefrom.